I replaced a Honeywell round thermostats for a programmable digital one. The heater and the AC both will turn on. The blower turns on when the heater is on, but the blower doesn't turn on when the AC turns on. The Fan "on and auto" switch when turned to the on position will not turn on the fan. The new thermostat is a Honeywell RTH221B. There are to sets of wires to the thermostat. One has 4 wires, Red, Green, Black and Wht. The other has 3 wires, Red, what and black. Why does the fan not turn on when the switch is in the on position and why does the blower not turn on when the AC turns on. I don't have either other black wires connect to anything. I believe the black wire coming from the set of 4 wires, which is hot when measured with a meter, is the fan wire but I'm not sure.

What were the wires hooked to when you took them off?

That's good question because I don't remember. I replaced it in the fall and never checked to see it the AC was working but the heater seemed to work fine. The installation guide stated to tape off the wires that were hooked to C, X1 and B. I think the black wires were hook to C and X1 but I not 100% sure. Would be any harm in connecting the black wire that is hot just like the green wire to the "G" or fan terminal to see if it the fan on swith works?

Sorry for the spelling errors. I type faster then I think.

Go to your furnace take the bottom door off and write down what color wires go to what letter on the control board. This is the best way to be sure

I will do that tomorrow. For now I'm just turning on the blower manually to circulate the AC air so it doesn't get to hot in the house.